"You can quickly see that there are ways now to get in and mess with [electrical] power if you have access to it."

Alexander went so far as to warn that certain cyber threats were outranked only by nuclear bombs or weapons of mass destruction.

As such, the U.S. - which is currently formulating an "active defense" strategy - will respond to attacks in the digital realm as it would in the real world.

However, Alexander acknowledged that current cyber defenses remain "far from adequate."

"In cyber, we have not solved the defensive portion... From my perspective, there is a lot that we can do to fix that before we take offensive actions. Whether or not we do that, it's coming. It's [simply] a question of time.

"People say, 'Aw that's five years out, it's two years out.' What we don't know is how far out it is, an attack in cyberspace, and what that will be? Will it be against commercial infrastructure, government networks? Will it be against platforms? We [just] don't know," he added.